Method for thrombogenicity testing of implanted medical device

ABSTRACT

A method of determining the thrombogenicity of an implantable medical device is disclosed. The implanted device is exposed in vitro to platelet rich plasma, the activity of an indicator is assayed, and the thrombogenicity is determined.

RELATED APPLICATIONS

This application is a continuation of co-pending U.S. application Ser.No. 14/256,527, filed Apr. 18, 2014, the entirety of which isincorporated herein by reference.

FIELD OF THE DISCLOSED SUBJECT MATTER

The disclosed subject matter relates to apparatus and techniques fortesting the effects of medical implantation in a subject. Particularly,the present disclosed subject matter is directed to devices, methods,and systems for determining the thrombogenicity characteristics of animplanted medical device in the circulatory system.

BACKGROUND

A leading cause of mortality with the developed world is cardiovasculardisease. Coronary disease is of significant concern. Patients havingsuch disease have narrowing in one or more coronary arteries. Generally,however, patients have narrowing in multiple coronary arteries. Onetreatment for the narrowing is stenting the blood vessel. Stentinginvolves the placement of a stent at the site of the artery closure.This type of cardiac intervention has proved effective in restoringvessel patency and decreasing myocardial ischemia. However the exposureof currently used metallic stents to flowing blood can result inthrombus formation, smooth muscle cell proliferation and acutethrombotic occlusion of the stent.

Drug eluting stents (“DES”) generally result in lower restenosis andrevascularization rates as compared to bare metal stents especially invessels having a diameter greater than approximately 3.0 mm (“largevessels”).

A safety concern associated with drug-eluting stents is the occurrenceof stent thrombosis (“ST”), a condition that occurs when a blood clot orthrombus forms on the surface of a stent, raising the risk of reductionof blood flow or vessel closure. If thrombus forms, the complicationscan include recurrent chest pain or heart attack. It is understood thatST can occur within 24-48 hours after deployment of the stent, referredto as “acute thrombosis,” The occurrence of ST within or year afterdeployment is referred to as “late thrombosis,” ST which occurs morethan one year after deployment is referred to as “very late thrombosis.”

“Thrombogenicity” refers to the tendency of a vascular implant or othermaterial such as a stents, embolic protection devices, artificialvalves, drug coated balloons, guidewires, or other percutaneouslyintroduced device, to produce a thrombus in contact with the blood.Furthermore, ST could lead to peripheral embolization caused by thrombidetached from the stent. Thrombogenicity resulting from stentimplantation is influenced by several factors: (a) blood borne factors,(b) stent related factors, and (c) vessel wall factors.

Blood borne factors include the activity of platelets and fibrinogen inthe bloodstream, etc. Both platelet adhesion and fibrin deposition areconsidered a significant step in thrombus formation. Thesefactors—platelet adhesion and fibrin deposition—occur simultaneously ina positive feed back mechanism. In other words, the occurrence of onefactor reinforces the occurrence of the other factor. For instance,contact with a foreign surface induces platelet activation. Activatedplatelets attach and detach and roll before ultimately forming stableadhesive interactions. Activated platelets also bind soluble fibrinogen,which leads to platelet aggregation. The deposition of insoluble fibrinresults in more platelet adhesion, more fibrin deposition and growth ofthrombus. The same processes can also be triggered by an initialfibrinogen adsorption to the foreign surface, followed by plateletactivation, adhesion, aggregation, fibrin deposition and thrombusformulation.

Stent related factors, such as, the material front which the stent isconstructed, the design of the stent, the type of surface of the stent,and stent apposition can influence the thrombogenicity of a stent.Thrombogenicity can also be influenced by vessel wall factors, such astissue factor, plaque material and/or plaque rupture, and vessel wallinflammation, etc. Stent thrombosis can occur despite anti-coagulativetreatment particularly on stents of poor biocompatibility.

While vessel wall factors are necessarily considered by the physicianduring an evaluation of the patient's condition, it is possible to testthe blood borne and stent related factors in vitro. Current techniquesfor thrombogenicity testing in whole blood include Chandler Loop and the“rocker method.” According to such techniques, stents were evaluated byfirst weighing each unit prior to experimentation, incubating with wholeporcine blood in devices in tubing/test tubes in Chandler loop ormechanical rocker for approximately ninety minutes at 37° C., andsubsequently washing, drying and weighing the stents to determine netweight gain, which is related to thrombus size. This approach toevaluating the thrombogenicity of the stent configuration has severaldisadvantages. One disadvantage, for example, is that the measurement ofthe weight gain by the medical device is typically highly variable.Moreover, the correlation between the weight gain of the device and itsassociated thrombogenicity has not been found to be reliably related tothe size of the device. Consequently, results of this test for one typeof device are not broadly comparable to results for other devices thatare different, e.g., larger or smaller than the tested device.

Due to the risk of acute ST, it is useful to provide a technique whichallows for a sensitive and reproducible determination of thethrombogenic potential of implanted medical devices, e.g., coronarystents, embolic protection devices and biosorbable coronary scaffolds.

SUMMARY OF THE DISCLOSED SUBJECT MATTER

The purpose and advantages of the disclosed subject matter will be setforth in and apparent from the description that follows, as well as willbe learned by practice of the disclosed subject matter. Additionaladvantages of the disclosed subject matter will be realized and attainedby the methods and devices particularly pointed out in the writtendescription and claims thereof, as well as from the appended drawings.

A method for determining the thrombogenicity of an implanted medicaldevice is provided including deploying the medical device in a testingdevice; exposing the medical device to platelet rich plasma (PRP) in thetesting device; determining activity of an indicator, and determiningthrombogenicity of the medical device based on the indicator.

In some embodiments, the medical device is a stent, embolic protectiondevice or a coronary scaffold.

In some embodiments, the testing device includes tubing, and deployingthe medical device includes deploying the medical device in the tubingor test tube. In some embodiments, exposing the medical device to PRPincludes rotating the tubing or test tube. In some embodiments, exposingthe medical device to PRP includes providing a flow of PRP through thetubing.

In some embodiments, determining activity of an indicator includesassaying the amount of LDH associated with the medical device. In someembodiments, determining activity of an indicator includes assaying theamount of D-dimer associated with the medical device.

In some embodiments, exposing the medical device to PRP in the testingdevice comprises exposing the medical device to human PRP. In someembodiments, exposing the medical device to PRP in the testing devicecomprises exposing the medical device to porcine PRP.

A method for evaluating thrombogenicity an implanted medical device isprovided including deploying the medical device in a testing devicehaving a first parameter, exposing the medical device to platelet richplasma (PRP) in the testing device, determining activity of anindicator, and determining thrombogenicity of the medical device basedon the indicator. The first parameter of the medical device is changed,and the steps deploying the medical device in a testing device, exposingthe medical device to PRP, determining activity of an indicator, anddetermining thrombogenicity of the medical device based on the indicatorare repeated with the modified first parameter.

In some embodiments, the first parameter is the percentage of themedical device that is uncoated. In some embodiments, the firstparameter is a coating of the medical device. In some embodiments, thefirst parameter is the percentage of polymer to medication in a coatingof the medical device.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and are intended toprovide further explanation of the disclosed subject matter claimed.

The accompanying drawings, which are incorporated in and constitute partof this specification, are included to illustrate and provide a furtherunderstanding of the method and device of the disclosed subject matter.Together with the description, the drawings serve to explain theprinciples of the disclosed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the drug delivery device inaccordance with the disclosed subject matter.

FIG. 2 is a schematic representation of an alternative geometry of astent in accordance with the disclosed subject matter.

FIG. 3 is a schematic representation of a stent delivery system inaccordance with the disclosed subject matter.

FIG. 4 illustrates a method for performing a thrombogenicity test inaccordance with the disclosed subject matter.

FIGS. 5-7 illustrate an apparatus for performing the method inaccordance with the disclosed subject matter.

FIG. 8 illustrates a lower power SEM of a medical device afterperforming a portion of the method in accordance with the disclosedsubject matter.

FIG. 9 illustrates a medium power SEM of a medical device afterperforming a portion of the method in accordance with the disclosedsubject matter.

FIG. 10 illustrates a high power SEM of a medical device afterperforming a portion of the method in accordance with the disclosedsubject matter.

FIG. 11 illustrates results of a portion of the method in accordancewith the disclosed subject matter.

FIG. 12 illustrates comparative results on several medical devices ofthe method in accordance with the disclosed subject matter.

FIG. 13 illustrates a further method for performing a thrombogenicitytest in accordance with the disclosed subject matter.

FIG. 14 illustrates comparative results on several medical devices ofthe method of FIG. 13 in accordance with the disclosed subject matter.

FIGS. 15-16 illustrate an apparatus for performing the method inaccordance with the disclosed subject matter.

FIG. 17 illustrates comparative results on several medical devices ofthe method in accordance with the disclosed subject matter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to various embodiments of thedisclosed subject matter, an example of which is illustrated in theaccompanying drawings. The method and corresponding steps of thedisclosed subject matter will be described in conjunction with thedetailed description of the device.

The methods and devices presented herein are directed to technique andapparatus for testing the thrombogenetic potential of implanted medicaldevices.

For purpose of explanation and illustration, thrombogenicity testing canbe performed on devices such as stents illustrated in FIGS. 1-3 and isdesignated generally by reference character 100. The device 100generally includes an intraluminal base stent, including a stent body10. As illustrated in the various embodiments shown in FIGS. 1-2, thestent can be configured in a variety of geometries. Although the deviceand methods associated with the present subject matter can be used invessels of any size, for purposes of explanation and not limitation, thepresent disclosure discusses a stent suitable for use in small vessels,e.g., vessels having a diameter of less than or equal to approximately3.0 mm and an axial length of approximately 12 mm. Prior to deploymentthe stent is crimped on a balloon, or other suitable expandable device.Crimping can be performed by pressurizing the balloon while the stent isradially compressed onto the balloon with a crimping apparatus. Once thestent has reached its radially compressed configuration, the pressurewithin the balloon can be released, while an inward crimping forceexerted on the stent by the crimping apparatus is maintained. After adwell time, the inward crimping force can be discontinued, and theballoon and crimped stent are removed from the crimping apparatus. As aresult of the crimping process, balloon material extends radiallyoutward through interstices of the stent to facilitate stent retentionon the balloon while advancing the stent delivery catheter through avessel lumen.

The expanded diameter of the stent ranges from about 2.25 mm at lowerballoon inflation pressures (e.g., about 8 atm) to about 2.59 mm athigher balloon inflation pressures (e.g., about 16 atm). In variousembodiments, the base stent is designed for use in small vessels havingdiameters of greater than or equal to approximately 2.25 mm to 2.5 mm.The stent body 10 is preferably but not necessarily balloon expandableand can be fabricated from any suitable metallic material including,e.g., stainless steel, tantalum, nickel-titanium, cobalt-chromium,titanium, shape memory and superelastic alloys, and the noble metalssuch as gold or platinum, as described in U.S. Pat. No. 6,939,373, whichis herein incorporated by reference in its entirety. Alternatively, aself-expanding stent can be employed wherein the stent automaticallyexpands at the desired location within the lumen by retracting a sheathon the delivery catheter. In some embodiments, the stent body isfabricated from L-605 cobalt chromium (CoCr) alloy. In otherembodiments, the stent body 10 can be described more particularly ashaving a series of interconnected strut members which define a pluralityof first peaks, second peaks, and valleys disposed therebetween.Although the stent is not divided into separate elements, for ease ofdiscussion references to peaks and valleys is appropriate. The number ofpeaks and valleys can vary in number for each ring depending upon theapplication. Thus, for example, if the stent is to be implanted in acoronary artery, a lesser number of peaks and valleys are required thanif the stent is implanted in a peripheral artery, which has a largerdiameter than a coronary artery.

Such a small-vessel stent is used in patients who have narrowing insmall coronary arteries that are greater than or equal to 2.25 mm toless than or equal to 2.50 mm in diameter and where the affected lengthof the artery is less than or equal to 28 mm long.

As shown in FIGS. 1-2, stent body 10 is made up of a plurality ofcylindrical rings 15 which extend circumferentially around the stentwhen it is in a tubular form. The stent has a delivery catheter outershaft diameter of 0.032″ distally and 0.026″ proximally. Eachcylindrical ring has a cylindrical ring proximal end and a cylindricalring distal end. Typically, since the stent is laser cut from a tubethere are no discreet parts such as the described cylindrical rings andlinks. However, it is beneficial for identification and reference tovarious parts to refer to the cylindrical rings and links and otherparts of the stent as follows.

Each cylindrical ring 15 defines a cylindrical plane which is a planedefined by the proximal and distal ends of the ring and thecircumferential extent as the cylindrical ring travels around thecylinder. Each cylindrical ring includes cylindrical outer wall surfacewhich defines the outermost surface of the stent, and cylindrical innerwall surface which defines the innermost surface of the stent. Thecylindrical plane follows the cylindrical outer wall surface.

In keeping with the invention, an undulating link is positioned withincylindrical plane. The undulating links connect one cylindrical ring 15to an adjacent cylindrical ring 15 and contribute to the overalllongitudinal flexibility to the stent due to their unique construction.The flexibility of the undulating links derives in part from curvedportion 16 connected to straight portions 18. In the exemplaryembodiment shown in FIG. 1, the straight portions are substantiallyperpendicular to the longitudinal axis of the stent. Thus, as the stentis being delivered through a tortuous vessel, such as a coronary artery,the curved portions 16 and straight portions 18 of the undulating linkswill permit the stent to flex in the longitudinal direction whichsubstantially enhances delivery of the stent to the target site. Thenumber of bends and straight portions in a link can be increased ordecreased from that shown, to achieve differing flexibilityconstructions. With the straight portions being substantiallyperpendicular to the stent longitudinal axis, the undulating link actsmuch like a hinge at the curved portion to provide flexibility. Astraight link that is parallel to the stent axis typically is notflexible and does not add to the flexibility of the stent.

The stent body 10 can be described more particularly as having aplurality of peaks 20 and valleys 22, as shown in FIG. 2. Although thestent is not divided into separate elements, for case of discussionreferences to peaks and valleys is appropriate. Each of the cylindricalrings 15 has a plurality of peaks 20 which have struts 18 attached to anapex. The struts can be either curved or straight depending upon theparticular application.

The stent body 10 can be made in many ways. One exemplary method ofmaking the stent is to cut a thin-walled tubular member, and to removeportions of the tubing in the desired pattern for the stent, leaving. Insome embodiments, the tubing is cut in the desired pattern by means of amachine-controlled laser as is well known in the art. In someembodiments, the struts have a thickness of less than approximately 110μm. In a specific embodiment, the struts have a thickness of 81 μm.

In some embodiments, the base stent is uncoated, also referred to as a“bare metal stent” (BMS). In some embodiments, the base stent is coatedwith active and inactive ingredients.

The inactive ingredient(s) include polymers, e.g.,poly(N-acetylglucosamine) (Chitin), Chitosan, poly(3-hydroxyvalerate),poly(D,L-lactide-co-glycolide), poly(1-lactide-co-glycolide)poly(3-hydroxybutyrate), poly(4-hydroxybutyrate),poly(3-hydroxybutyrate-co-3-hydroxyvalerate), polyorthoester,polyanhydride, poly(glycolic acid), poly(glycolide), poly(L-lacticacid), poly(L-lactide), poly(D,L-lactic acid), poly(D,L-lactide),poly(L-lactide-co-D,L-lactide), poly(caprolactone),poly(L-lactide-co-caprolactone), poly(D,L-lactide-co-caprolactone),poly(glycolide-co-caprolactone), poly(trimethylene carbonate), polyesteramide, poly(glycolic acid-co-trimethylene carbonate),co-poly(ether-esters) (e.g. PEO/PLA), polyphosphazenes, biomolecules(such as fibrin, fibrin glue, fibrinogen, cellulose, starch, collagenand hyaluronic acid, elastin and hyaluronic acid), polyurethanes,silicones, polyesters, polyolefins, polyisobutylene andethylene-alphaolefin copolymers, acrylic polymers and copolymers otherthan polyacrylates, vinyl halide polymers and copolymers (such aspolyvinyl chloride), polyvinyl ethers (such as polyvinyl methyl ether),polyvinylidene halides (such as polyvinylidene chloride),poly(vinylidene fluoride), poly(vinylidenefluoride-co-hexafluoropropylene), polyacrylonitrile, polyvinyl ketones,polyvinyl aromatics (such as polystyrene), polyvinyl esters (such aspolyvinyl acetate), acrylonitrile-styrene copolymers, ABS resins,polyamides (such as Nylon 66 and polycaprolactam), polycarbonatesincluding tyrosine-based polycarbonates, polyoxymethylenes, polyimides,polyethers, polyurethanes, rayon, rayon-triacetate, cellulose, celluloseacetate, cellulose butyrate, cellulose acetate butyrate, cellophane,cellulose nitrate, cellulose propionate, cellulose ethers, carboxymethylcellulose, fullerenes and lipids. In a specific embodiment, the inactiveingredients are the polymers poly n-butyl methacrylate (PBMA) andPVDF-HFP, which is comprised of vinylidene fluoride andhexafluoropropylene monomers. PVDF-HFP is a non-erodiblesemi-crystalline random copolymer with a molecular weight of 254,000 to293,000 daltons. PBMA is a homopolymer with a molecular weight of264,000 to 376,000 daltons.

The active ingredient(s) can include a therapeutic agent that caninclude any substance capable of exerting a therapeutic or prophylacticeffect. Examples of therapeutic agents include antiproliferativesubstances such as actinomycin D, or derivatives and analogs thereof.Synonyms of actinomycin D include dactinomycin, actinomycin IV,actinomycin 11, actinomycin X1, and actinomycin C1. The bioactive agentcan also fall under the genus of antineoplastic, anti-inflammatory,antiplatelet, anticoagulant, antifibrin, antithrombin, antimitotic,antibiotic, antiallergic and antioxidant substances.

Examples of such cytostatic or antiproliferative agents includeangiopeptin, angiotensin converting enzyme inhibitors, calcium channelblockers, colchicine, proteins, peptides, fibroblast growth factor (FGF)antagonists, fish oil (omega 3-fatty acid), histamine antagonists,inhibitors of HMG-CoA reductase, monoclonal antibodies (such as thosespecific for Platelet-Derived Growth Factor (PDGF) receptors),nitroprusside, phosphodiesterase inhibitors, prostaglandin inhibitors,suramin, serotonin blockers, steroids, thioprotease inhibitors,triazolopyrimidine (a PDGF antagonist), and nitric oxide. An example ofan antiallergic agent is permirolast potassium. Other therapeuticsubstances or agents which can be appropriate agents include cisplatin,insulin sensitizers, receptor tyrosine kinase inhibitors, carboplatin,alpha-interferon, genetically engineered epithelial cells, steroidalanti-inflammatory agents, non-steroidal anti-inflammatory agents,antivirals, anticancer drugs, anticoagulant agents, free radicalscavengers, estradiol, antibiotics, nitric oxide donors, super oxidedismutases, super oxide dismutases mimics,4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO),tacrolimus, dexamethasone, ABT-578, clobetasol, cytostatic agents,prodrugs thereof, co-drugs thereof, and a combination thereof.

In a specific embodiment the active agent is a proliferation signalinhibitor, or mTOR inhibitor such as a semi-synthetic macrolideimmunosuppressant which has been shown to inhibit in-stent neointimalgrowth in coronary vessels following stent implantation due to itsanti-proliferative properties.

In some embodiments, PBMA, which adheres well with metallic materialsand other polymers, is used as a primer to coat the base stent. PVDF-HFPis used as a drug matrix that is mixed with the therapeutic agent. ThePVDF-HFP/therapeutic agent mixture is adhered to the surface of the PBMAcoated stent. In a specific embodiment, this PVDF-HFP/therapeutic agentmixture comprises 83% polymer and 17% therapeutic agent. The thicknessof the polymer coating is less than approximately 10 μm. In a specificembodiment, the thickness of the polymer coating is 7.1 μm. Theconcentration of the therapeutic agent in the copolymer is about 50μg/cm² to about 150 μg/cm². In a specific embodiment the concentrationof the therapeutic agent in the copolymer is 100 μg/cm². Systems andmethods for coating stents are disclosed in U.S. Pat. No. 8,003,157,which is herein incorporated by reference.

In some embodiments, thrombogenicity testing is performed onbioresorbable scaffolds.

The techniques described herein incorporate the use of platelet richplasma (PRP) instead of whole blood. Platelets are indispensableinitiators of thrombosis and their adhesion to intravascular devices isthe critical step in the thrombus formation. In some embodiments, humanPRP is used. In some embodiments porcine PRP is used.

An exemplary testing method 200 is illustrated in FIG. 4. At step 202,PRP is obtained from whole blood. In the case where use of porcine PRPis desired, fresh heparinized porcine blood is first obtained. The bloodis spun at 350×g for 15 minutes at 20° C. Accordingly, the PRP isseparated from the blood cell pellet consisting of red and white bloodcells. Subsequently, PRP is spun at 54×g for 10 minutes at 20° C. toremove residual red blood cells. The platelet number is determined usingthe Coulter particle counter.

In this method, medical devices, such as metallic coronary stents or BVSscaffolds are deployed in a test compartment at step 204. In someembodiments, the medical device is deployed in a Chandler loop apparatus300, as illustrated in FIGS. 5-7. FIG. 5 illustrates a tubing loop 302,e.g., fabricated from a silicone material, to be used in connection withthe Chandler loop apparatus. The medical device M is loaded into thesilicon tubing 302. In some tests, several medical devices are loadedinto tubing. For example, six stents can be loaded into each tubing loop302.

A subsequent step in the process is to perfuse the medical device(s) Mwith PRP (step 206). Each tubing loop 302 having the medical device(s) Mis filled with freshly prepared PRP, and loaded onto the drum 304 of theChandler loop apparatus 300, as illustrated in FIG. 6. Once loaded ontothe drum 304, the loops 302 are rotated by the drum 304 for a period ofabout 2 hours at 37 C at 31 rpm that generates 100 mL/min PRP continuousflow rate At the end of the 2 hours, the PRP is removed from the tubing302 and rinsed with 5 mL of phosphate buffered saline (PBS). Asillustrated in FIG. 6, the tubing segment 302 containing the medicaldevice M is cut at locations 308 adjacent each end of the device inorder to remove it from the tubing 302 for analysis. FIGS. 8-10illustrate a medical device, i.e., a Vision® stent (Abbott Vascular)after Chandler loop perfusion with human PRP. FIG. 8 is a low power SEMof the stent, with adherent platelets (denoted by arrows “P”) andpatches of fibrin deposits (denoted by arrows “F”). Similarly, a mediumpower SEM (FIG. 9) and high power SEM (FIG. 10) illustrate platelets andfibrin deposits.

A subsequent step in the process is to evaluate the thrombogenicpotential based on platelet adhesion to the deployed units. For example,the activity of an indicator on the medical device is determined (step208). In some embodiments, lactate dehydrogenase (LDH) activity ismeasured to determine thrombogenicity. The extent of platelet adhesionis determined by measuring the LDH activity extracted from the adherentplatelets which is directly proportional to the number of platelets. AnLDH elution procedure is performed to extract adherent platelets fromthe stent by lysing with 1% Triton-X100 for 1 hour at 37° C., including20 minutes in an ultrasonic bath followed by rotation for 40 minutesusing 20 cycle/minute. LDH activity was analyzed in the platelet lysateusing commercial assay kit (CytoTox 96 Assay Kit for LDH, Promega). LDHactivity in platelets isolated from PRP is illustrated in FIG. 11.

A subsequent step in the process is to perform a statistical analysis ofthe results (step 210). The mean activity in LDH micro units for a setof 6 BMS stents is determined, using the BMS mean LDH activity as athrombogenicity standard. The mean LDH activity is determined for anyset of test samples and normalized against BMS thrombogenicity standard.For example, the individual LDH absorbance micro units from the testedstents and scaffolds, including BMS standards, is recorded. Each LDHabsorbance micro unit value is divided by the nominal surface area inmm² to determine absorbance micro units per surface area. An interquartile test for each set of six stents/scaffolds as well as BMSstandards is conducted. Outliers are removed and the average LDHabsorbance is determined. Thrombogenicity is reported in LDH micro unitsper stent/scaffold surface area (mm²) with the standard deviation (SD)and coefficient of variation (CV) for each set of six stents orscaffolds. The method included the following parameters for sensitivity,specificity, and reproducibility. Sensitivity, expressed as LOD and LLOQwas 1.3 and 3.0 million platelets per mL of stent elute, respectively.Specificity was demonstrated by consistent uniformity of plateletpopulations found in PRP preparations and by the absence of any otherblood cells. Regarding reproducibility, intra and inter assay precisionwas below 30% CV, varying between 14 and 25%. See Table I below:

TABLE 1 Analysts Analyst #1 Analyst #2 Type of Mean Mean Stent Lot #LDH/mm² SD % CV N LDH/mm² SD % CV N BMS 1 2.4 0.34 15 8 2.5 0.35 14 8BMS 2 3.6 0.87 25 6 3.4 0.85 25 6 Scaffold 3 2.1 0.43 21 12 2.0 0.35 1812

FIG. 12 illustrates the thrombogenicity potential for three products.Thrombogenicity was highest for the BMS, followed by a coated stent,followed by a coated scaffold.

A method for evaluating and designing implanted medical devices havingcoatings is also described herein. For example, the novel testingtechniques provided herein enable the testing of thrombogenicity ofvarious devices to determine their suitability for implantation in apatient. In an exemplary embodiment, as illustrated in FIG. 13, a methodfor evaluating a medical device is provided. As with the method 200described herein above, PRP is obtained from whole blood (step 502). Amedical device, having a first parameter, is deployed in a testcompartment (step 504). The first parameter can refer to characteristicsof the coating of the stent or scaffold being evaluated. For example,one parameter can include a composition of the coating, e.g., themedication that is to be released at the location of the stent orscaffold. In another example, the parameter can include the percentageof the stent of scaffold that is coated with medication. (A BMS wouldhave 0% coated. A nominally “coated” stent can have, e.g., 5-15%uncoated, due to cracks in the coating or a result of the manufacturingprocess.) In a further example, the parameter can include the percentageof the coating that comprises medication and the percentage of thecoating that comprises a polymer. In another example, the parameter caninclude the rate of release of the medication from the coating.

At step 506, the medical device is exposed to PRP, for example, usingthe Chandler loop process described herein above. At the step 508, theactivity of an indicator, such as LDH is determined, and statisticalanalysis can be performed at step 510. Having evaluated thethrombogenicity of the medical device with the first parameter, thefirst parameters can be varied (step 512), typically in connection withanother sample of the medical device. For example, as discussed above,the composition of the coating can be changed, the percentage coatedarea can be changed, the ratio of medication to polymer can be varied,etc. The process continues at step 504, in which the medical device isdeployed in the test compartment and exposed to PRP (step 506). As aresult of the method described herein above, the effect of certainparameters on thrombogenicity can be established. It is understood thatstep 512 can occur after steps 504-510 are performed, or concurrentlywith steps 504-510. In other words, medical devices with the parametersat a plurality of different selections can be evaluated simultaneously.

In an example, illustrated in FIG. 14, BVS scaffolds with 5 and 10% ofuncoated area were not statistically different from nominal BVSscaffolds with 2.5% uncoated area. BVS units with cracks hadstatistically similar thrombogenicity results as units with 5% and 10%uncoated area. As a result of this analysis, it was understood thatcompromised area/coating integrity specification could be increased from2.5% to 5.0% for the BVS scaffolds, with similar results from athrombogenicity standpoint.

In another exemplary embodiment, a method is described herein andillustrated in FIGS. 15-16. According to this method, testing apparatusand methods are provided to mimic in vivo conditions; to eliminatepotential effects of a coating material released into PRP, and to allowfor direct evaluation of anti-thrombogenic effects of the coatingmaterial on the stent surface. As illustrated in FIGS. 15 and 16,medical devices M, such as tents, are deployed into tygon tubing 602within test apparatus 600, and perfused with a continuous flow of PRP inthe direction of arrow F.

As an alternative, or in addition to LDH testing, determination offibrin deposits can be performed by D-dimer method with reagents forhuman fibrin, since D-dimer is a final product of plasmin induceddegradation of fibrin deposits. Accordingly, combination of LDH andD-dimer measurements can be used for a thrombogenicity assessment. Forexample, using the apparatus 600, four stents can be perfused with PRPfrom a single bag 604 at 37° C. for 60 minutes. As described hereinabove, after perfusion LDH and D-dimer are measured to evaluatethrombogenicity of the stents M.

FIG. 17 illustrates testing performed on a number of medical devices,including BMS and coated stents. In these tests, the flow-throughapparatus 700 described above was used to perfuse human PRP on thedevices. For each test, LDH and D-dimer were measured on a BMS andexpressed as 100% standard (denoted 750 in FIG. 17). For the coatedstents, PRP was perfused, and LDH and D-dimer concentrations werenormalized as a percentage to the LDH measurements of the BMS stents.The LDH concentrations are denoted as 760, and the D-dimerconcentrations are denoted as 770 in FIG. 17.

While the disclosed subject matter is described herein in terms ofcertain preferred embodiments, those skilled in the art will recognizethat various modifications and improvements can be made to the disclosedsubject matter without departing from the scope thereof. Moreover,although individual features of one embodiment of the disclosed subjectmatter can be discussed herein or shown in the drawings of the oneembodiment and not in other embodiments, it should be apparent thatindividual features of one embodiment can be combined with one or morefeatures of another embodiment or features from a plurality ofembodiments.

In addition to the specific embodiments claimed below, the disclosedsubject matter is also directed to other embodiments having any otherpossible combination of the dependent features claimed below and thosedisclosed above. As such, the particular features presented in thedependent claims and disclosed above can be combined with each other inother manners within the scope of the disclosed subject matter such thatthe disclosed subject matter should be recognized as also specificallydirected to other embodiments having any other possible combinations.Thus, the foregoing description of specific embodiments of the disclosedsubject matter has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit thedisclosed subject matter to those embodiments disclosed.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the method and device of thedisclosed subject matter without departing from the spirit or scope ofthe disclosed subject matter. Thus, it is intended that the disclosedsubject matter include modifications and variations that are within thescope of the appended claims and their equivalents.

What is claimed is:
 1. A method for determining thrombogenicity of amedical device, comprising: deploying a medical device in a testcompartment; flowing a volume of platelet rich plasma (PRP) through thetest compartment; measuring an activity of a thrombogenic indicator onthe medical device; and determining a level of thrombogenicity of themedical device based on the activity of the thrombogenic indicator. 2.The method of claim 1, wherein the thrombogenic indicator includes oneor more of lactate dehydrogenase (LDH) or D-dimer extracted fromadherent platelets on the medical device.
 3. The method of claim 2,wherein measuring the activity of the thrombogenic indicator includesassaying an amount of LDH extracted from the adherent platelets.
 4. Themethod of claim 3, wherein the PRP includes the LDH in the platelets,and wherein measuring the activity of the LDH includes disrupting theadherent platelets to release the LDH.
 5. The method of claim 4, whereina level of the LDH released from the disrupted adherent plateletsindicates the level of thrombogenicity of the medical device.
 6. Themethod of claim 2, wherein measuring the activity of the thrombogenicindicator includes assaying an amount of D-dimer extracted from theadherent platelets.
 7. The method of claim 1, wherein the medical deviceis an intravascular device.
 8. The method of claim 7, wherein themedical device is a coronary stent.
 9. The method of claim 1, whereinthe test compartment includes a closed system.
 10. The method of claim9, wherein the closed system includes one or more of a tubing loop or atest tube.
 11. The method of claim 1, wherein flowing the volumeincludes a continuous flow of PRP through the test compartment.
 12. Themethod of claim 1, wherein flowing the volume includes a pulsatile flowof PRP through the test compartment to mimic in vivo conditions.
 13. Themethod of claim 1, wherein the volume of PRP includes mammalian PRP. 14.The method of claim 13, wherein the mammalian PRP includes human PRP.15. The method of claim 13, wherein the mammalian PRP includes porcinePRP.
 16. A method for evaluating thrombogenicity of a medical device,comprising: (a) deploying a medical device in a test compartment,wherein the medical device includes a parameter of a coating compositionon the medical device; (b) flowing a volume of platelet rich plasma(PRP) through the test compartment; (c) measuring an activity of athrombogenic indicator on the medical device; (d) determining a level ofthrombogenicity of the medical device based on the activity of thethrombogenic indicator; and (e) modifying the parameter and repeatingoperations (a)-(d) using the modified parameter.
 17. The method of claim16, wherein the parameter is a measure of coated surface area on themedical device, and wherein the coating composition includes amedication.
 18. The method of claim 16, wherein measuring the activityof the thrombogenic indicator includes assaying an amount of one or moreof lactate dehydrogenase (LDH) or D-dimer extracted from adherentplatelets on the medical device.
 19. The method of claim 16, wherein thetest compartment includes a closed system.
 20. The method of claim 16,wherein flowing the volume includes a continuous flow of PRP through thetest compartment.